The secretion of parathyroid hormone (PTH) by parathyroid gland is directly regulated by extracellular calcium ions (Ca2+) via the Ca2+ sensing receptor. However, there are reports of magnesium (Mg2+) effect on PTH secretion: acute hypermagnesemia can suppress PTH, and it is suggested that the mechanism of suppression is similar to that of hypercalcemia. During platelet apheresis, the infused citrate forms complexes with both Ca2+and Mg2+ ions and, therefore, decreases the concentration of the bioactive forms (ionized Ca and Mg) of both cations in the blood returned to the donor. The resulting hypocalcemia and concomitant hypomagnesemia could influence the PTH response and could be responsible for the citrate toxicity symptoms observed during apheresis. We are investigating the time course of changes in ionized Mg and Ca concentrations that occur during large volume leukapheresis procedures and their relation to the citrate toxicity symptoms. The apparent dissociation constant of the magnesium-ATP complex, KDMgATP, is used for determination of intracellular free magnesium with 31P NMR spectroscopy. We determined the constant experimentally by measuring the chemical shift difference between a- and b-ATP in the phosphorus (P) spectrum with 31P NMR spectroscopy and by determining the concentration of free magnesium using the fluorescence indicator Mag- fura-2. The value of KDMgATP = 8.2 ? 0.5 mM at 37 oC, pH = 7.2 and ionic strength 0.15 was verified by calculation using the concentration of total magnesium determined with the atomic absorption spectroscopy. The value of KDMgATP is being used to determine the relationship between red blood cell free Mg and serum ionized Mg.